1. Field of the Invention
The present invention relates generally to optical data storage. In particular, synchronizing data sectors in a multilevel optical data storage system is disclosed.
2. Relationship to the Art
In order to increase the capacity and speed of optical data storage systems, multilevel optical data storage systems have been developed. Note that in this specification, the term xe2x80x9cmultilevelxe2x80x9d refers to more than two levels. In a traditional optical data storage system, reflectivity of the recording medium is modulated between two states. The density of data recorded on an optical recording medium can be increased by modulating the reflectivity of the optical recording medium into more than two states.
Binary optical data storage systems employ special synchronization patterns at various locations within each sector of data to determine the correct alignment for decoding data. One can choose synchronization patterns that violate some constraint of the underlying code so that they cannot appear as part of normal data. This minimizes the number of false detections of synchronization sequences. For example, in a system with a (d, k) run-length limit (RLL) binary code, at least d, and at most k, zero channel bits must appear between each pair of one channel bits. Thus one can use a synchronization pattern with a run of k+1 zero channel bits between a pair of one channel bits.
Alternatively, one can use an unusual synchronization pattern and some special procedures to guarantee that the data never contains the synchronization pattern. In a CD optical data storage system, data is encoded with eight-fourteen modulation (EFM) code, which is a (2,10) RLL code that encodes each data byte (eight bits) into a 14-bit codeword. The CD system writes the synchronization pattern {100000000001000000000010} at the start of each frame of data. This synchronization pattern contains two adjacent runs of 10 zero bits between pairs of one bits, making it distinct and easy for the CD reader to locate within a frame of data. Although it is unusual, it still obeys the run-length limits of the EFM code and hence could occur within a frame of random data. However, when encoding the user data, the CD system adds three merge bits in front of each 14-bit codeword in order to maintain RLL across pairs of codewords and to apply DC control. The three merge bits are chosen so that the synchronization pattern can never appear in the data sequence.
Since binary optical data storage systems use RLL codes, they can use simple synchronization patterns that violate the RLL constraints (usually the upper RLL). However, in a multilevel system that uses a code with an upper RLL, because of the finer spacing of levels, a synchronization pattern that violates the RLL constraint might be difficult to distinguish from a valid data sequence of the same length that contains a small transition between two short runs. For example, if there are eight levels (0 to 7) in the system and the upper RLL is 10 symbols, then the synchronization pattern {7000000000007} is very similar to {7000001000007}, whereas the equivalent binary case {7000007000007} of two short runs combined together is much more distinct.
Moreover, a multilevel system may not even use a code with an upper RLL. A multilevel system may use a trellis or MSN code (see, for example, copending U.S. patent application Ser. No. 09/369,746, entitled xe2x80x9cCoding System And Method For Partial Response Channelsxe2x80x9d filed Aug. 6, 1999 which is herein incorporated by reference, which is much more complex than a RLL code and may or may not have an upper run-length limit. Because of the complexity of trellis and MSN codes, it may not be straightforward to create a short synchronization pattern that is very distinct from every valid data sequence of the same length. A long synchronization pattern will result in a complex synchronization detector. In addition, a synchronization pattern for one code probably will not be a good choice of synchronization pattern for other codes.
In view of the foregoing, there is a need for methods for synchronizing data in a multilevel optical data storage system that can be used with a wide variety of codes, that do not depend on the code having an upper RLL, and that are computationally simple.
Accordingly, a system and method for providing synchronization in multilevel optical data storage systems is disclosed. It is computationally efficient and useful for a wide variety of codes. On the write side, the system inserts a synchronization pattern at the start or at various locations within a sector of data. On the read side, the system uses a synchronization detector to detect the synchronization pattern.
The synchronization pattern can be a pseudo-noise (PN) sequence with one or more marks per PN chip to compensate for blurring effects of intersymbol interference during writing and reading. On the read side, the A/D converter may sample one or more times per mark. The synchronization detector precisely locates the synchronization pattern within the sampled read signal by slicing the signal and correlating it with a modified version of the PN sequence. If a system uses a two-dimensional cross constellation to modulate user data, then the synchronization pattern can include invalid pairs of symbols. The synchronization detector can be a simple correlation detector or comparator. If a system uses a DC control (DCC) encoder, then the synchronization pattern can be any unusual or distinct sequence of symbols, including those mentioned above. The DCC encoder, which chooses merge symbols from a plurality of candidate merge symbols to interlace with blocks of user data in order to provide DC balance, can eliminate candidate merge symbols that would result in the synchronization pattern appearing within the data stream. Finally, if the system uses a timing-recovery circuit that has distinct acquisition and tracking stages, then the synchronization pattern can mark the transition from acquisition to tracking.
It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, a method, or a computer-readable medium that includes certain types of marks that enable reliable data storage and recovery. Several inventive embodiments of the present invention are described below.
In one embodiment, a synchronization mark is created on a multilevel data storage medium. Creating the mark includes generating a pseudo-noise sequence having a plurality of bits; converting each 0 bit of the pseudo-noise sequence to the lowest symbol that is written to the multilevel data storage medium; converting each 1 bit of the pseudo-noise sequence to the highest symbol that is written to the multilevel data storage medium; deriving a resulting sequence of symbols from the converted bits; and writing the resulting sequence of symbols to the multilevel data storage medium.
In one embodiment, a synchronization mark on a multilevel data storage medium is detected by generating a pseudo-noise sequence having a plurality of bits; converting the pseudo-noise sequence to an expected synchronization sequence that would result from reading a synchronization mark generated using the pseudo-noise sequence; correlating the expected synchronization sequence with a read sequence from the data storage medium to produce a correlation sequence of correlation sums; and detecting the peak value of the sequence of correlation sums.
In one embodiment, a synchronization mark on a multilevel data storage medium is created. Creating the mark includes generating a data sequence to be written to the multilevel data storage medium using a two dimensional cross constellation having a plurality of invalid symbol pairs; generating a synchronization sequence of symbols that includes a combination of the invalid symbol pairs; and writing the synchronization sequence to the multilevel data storage medium.
In one embodiment, a synchronization mark on a multilevel data storage medium is created. Creating the mark includes defining a synchronization sequence; generating a data sequence to be written to the multilevel data storage medium; dividing the data sequence into a series of data blocks having merge symbols inserted between the data blocks so that the resulting sequence does not include the synchronization sequence; and writing the synchronization sequence to the multilevel data storage medium.
In one embodiment, a synchronization detector for detecting a synchronization mark written to a multilevel data storage medium includes a pseudo-noise sequence generator for generating a pseudo-noise sequence having a plurality of bits; a processor configured to convert the pseudo-noise sequence to an expected synchronization sequence that would result from reading a synchronization mark generated using the pseudo-noise sequence; a correlator configured to correlate the expected synchronization sequence with a read sequence from the data storage medium to produce a correlation sequence of correlation sums; and a peak detector configured to detect the peak value of the sequence of correlation sums.
In one embodiment, a data writing system for writing data to a multilevel data storage medium includes a pseudo-noise sequence generator for generating a pseudo-noise sequence having a plurality of bits; a processor configured to convert each 0 bit of the pseudo-noise sequence to the lowest symbol that is written to the multilevel data storage medium and to convert each 1 bit of the pseudo-noise sequence to the highest symbol that is written to the multilevel data storage medium; and a multiplexer configured to mix the resulting sequence of converted symbols with data from a data source.
In one embodiment, a multilevel optical storage disc includes multilevel marks that encode data; and synchronization marks determined by: generating a pseudo-noise sequence having a plurality of bits; converting each 0 bit of the pseudo-noise sequence to the lowest symbol that is written to the multilevel data storage medium; converting each 1 bit of the pseudo-noise sequence to the highest symbol that is written to the multilevel data storage medium; and deriving a resulting sequence of symbols from the converted bits.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures which illustrate by way of example the principles of the invention.